The present invention relates to compositions of diene elastomers reinforced with a white or inorganic filler, intended particularly for the manufacture of tires or of semi-finished products for tires, in particular of treads for these tires.
It relates more particularly to the use, in such compositions, of bonding agents for coupling reinforcing inorganic fillers and diene elastomers.
It is generally known that in order to obtain the optimum reinforcement properties imparted by a filler, the latter should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. Now, such conditions may only be obtained insofar as the filler has a very good ability firstly to be incorporated into the matrix during mixing with the elastomer and to disagglomerate, and secondly to be dispersed homogeneously in this matrix.
It is fully known that carbon black has such abilities, which is generally not true of inorganic fillers, because, for reasons of mutual attraction, the inorganic filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. These interactions furthermore tend to increase the viscosity of the rubber compositions in the uncured state and therefore to make them more difficult to work (“processability”) than in the presence of carbon black.
Since fuel economies and the need to protect the environment have become priorities, it has however proved necessary to produce tires having reduced rolling resistance, without adversely affecting their wear resistance. This has been made possible in particular due to the discovery of new rubber compositions reinforced with specific inorganic fillers referred to as “reinforcing” fillers, which are capable of rivaling a conventional tire-grade carbon black from the reinforcing point of view, while offering these compositions a lower hysteresis, which is synonymous with lesser rolling resistance for the tires comprising them, and also very good grip properties on wet, snow-covered or icy ground.
Such rubber compositions comprising reinforcing inorganic fillers for example of the silica or alumina type have for example been described in the patents or patent applications EP 501 227 or U.S. Pat. No. 5,227,425, EP 735 088 or U.S. Pat. No. 5,852,099, EP 810 258 or U.S. Pat. No. 5,900,449, EP 881 252, WO99/02590, WO99/06480, WO00/05300, WO00/05301 and WO02/10269.
Mention will be made in particular of documents EP 501 227, EP 735 088 or EP 881 252, which disclose diene rubber compositions reinforced with precipitated silicas of high dispersibility, such compositions making it possible to manufacture treads having a significantly improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and wear resistance. Such compositions having such a compromise of contradictory properties are also described in application EP 810 258, with specific aluminas of high dispersibility as reinforcing inorganic fillers.
Although the use of these specific, highly dispersible inorganic fillers has reduced the difficulties of processing in the uncured state the rubber compositions that contain them, they are still more difficult to process than for compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also referred to as bonding agent, the function of which is to provide the bond between the surface of the particles of inorganic filler and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
The term (inorganic filler/elastomer) “coupling agent” is understood in known manner to mean a compound capable of establishing a sufficient chemical and/or physical connection between the inorganic filler and the diene elastomer; such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula “Y-T-X”, in which:                Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica);        X represents a functional group (“X” function) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulfur atom;        T represents a divalent organic group making it possible to link Y and X.        
The coupling agents must particularly not be confused with simple agents for covering the inorganic filler which, in known manner, may comprise the Y function which is active with respect to the inorganic filler but are devoid of the X function which is active with respect to the diene elastomer.
Coupling agents, in particular (silica/diene elastomer) coupling agents, have been described in a large number of documents, the best known being bifunctional organosilanes bearing three organoxysilyl (in particular alkoxysilyl) functions as “Y” function and, as “X” function, at least one function capable of reacting with the diene elastomer, such as, in particular, a sulfurised functional group (i.e. comprising sulfur).
Thus, it was proposed in patent applications FR 2 094 859 or GB 1 310 379 to use a mercaptoalkoxysilane coupling agent for manufacturing treads for tires. It was quickly shown, and is today well known, that mercaptoalkoxysilanes are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the very high reactivity of the thiol-type sulfur functions —SH (“X” functions), which very rapidly results, during the preparation of the rubber compositions, in an internal mixer, in premature vulcanization, this also being known as “scorching”, in very high viscosities in the uncured state, and, finally, in rubber compositions which are virtually impossible to work and process industrially. To illustrate this problem, mention may be made for example of documents FR 2 206 330, U.S. Pat. No. 3,873,489 and U.S. Pat. No. 4,002,594.
To overcome this drawback, it has been proposed to replace these mercaptoalkoxysilanes by polysulfurised alkoxysilanes, in particular bis-(alkoxysilylpropyl) polysulfides such as described in many documents (see for example FR 2 149 339, FR 2 206 330, U.S. Pat. No. 3,842,111, U.S. Pat. No. 3,873,489 or U.S. Pat. No. 3,997,581). Among these polysulfides, mention must be made in particular of bis-3-triethoxysilylpropyl tetrasulfide (abbreviated to TESPT) or bis 3-triethoxysilylpropyl disulfide (abbreviated to TESPD).
These polysulfurised alkoxysilanes, in particular TESPT, are generally considered as the products providing, for vulcanized rubber compositions comprising a reinforcing inorganic filler, in particular silica, the best compromise in terms of resistance to scorching, ease of processing and reinforcing ability. As such, they are the coupling agents which are most used today in rubber compositions for tires, even if they have the known drawbacks (see for example patents U.S. Pat. No. 5,652,310, U.S. Pat. No. 5,684,171, U.S. Pat. No. 5,684,172) of being relatively costly and, what is more, of needing to be used most frequently in relatively large quantities.